Pacific Gas and Electric Co. briefly raised the pressure on its San Bruno natural gas line to the brink less than two years before the explosion that killed eight people - an action experts call a "huge gamble" that they fear made the pipe more susceptible to failure.

A Chronicle investigation into events before the explosion led PG&E officials to reveal that for two hours on Dec. 9, 2008, the company intentionally boosted gas pressure to the maximum legal limit of 400 pounds per square inch. That was more pressure than PG&E has ever acknowledged using on the line, which it normally ran at 375 pounds per square inch.

The utility initially explained that it had boosted the pressure because federal regulations required it to do so, but later conceded that its interpretation was inaccurate. It then explained that the spike was "part of our operating practice."

This is the first time the company has ever acknowledged running the San Bruno line at its legal maximum - a level now under scrutiny by federal investigators in light of revelations that PG&E had erroneous records about the pipeline's characteristics.

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The next time the pressure exceeded 375 pounds per square inch on the line was on Sept. 9, when a malfunction caused a surge to 386 pounds - a spike that coincided with the deadly explosion and fireball in San Bruno that destroyed 38 homes.

Early strain in line

PG&E's intentional surge in 2008 could have strained the San Bruno line and made it more vulnerable to failure at lower pressure levels, according to experts interviewed by The Chronicle. Strain caused by one surge, they said, can weaken a pipe to the point where it can fail at a lower point when pressure surges again.

"If there was a defect very close to failure, it could cause that defect to enlarge," said Robert Eiber, a nationally recognized pipeline expert. "I'm frankly amazed they were willing to take the risk. I don't know if they were aware of the risk they were taking or not. But in a case like this, it was a huge gamble."

When queried by The Chronicle, PG&E initially said federal regulators require that a pipe be run at full strength at least once every five years in order for the utility to "preserve" its legal capacity.

If the pressure ever exceeds that limit, the utility is obligated to conduct a costly, high-priority inspection of the line.

Had PG&E not spiked the pressure on the 30-inch transmission line running from Milpitas to San Francisco, utility spokesman Denny Boyles first said, the pipe's capacity would have been permanently reduced to 375 pounds per square inch under federal law.

Changing stories

But the spokesman later backtracked when asked to cite the specific federal regulation, saying PG&E's earlier response was "too general and as a result inaccurate." He maintained that PG&E still believed that 400 pounds per square inch was a "very safe level" for the San Bruno line.

In a subsequent statement, the company no longer said federal law had prompted its action.

"Putting the pressure up to 400 was part of our operating practice," Paul Moreno, a PG&E spokesman, said Friday. He said the utility operates its lines at their maximum once every five years. He declined to elaborate.

Experts shocked

Although the San Bruno line did not fail during the 2008 pressure spike, experts interviewed by The Chronicle said the utility had been taking a terrible chance.

Richard Kuprewicz, a pipeline safety consultant in Redmond, Wash., said that in spiking the pressure, PG&E's action created the likelihood that "stable operations may be stressed and become unstable."

"You just don't go out there and do real-time pressure tests of this magnitude on lines without doing careful thought and evaluation," Kuprewicz said. "This is a gas transmission pipeline. This is in the middle of a city. You don't just go raise the pressure on pipelines and hope they stay together."

Eiber, a pipeline integrity consultant and researcher in Columbus, Ohio, with 50 years of experience in the business, said the natural gas industry all but abandoned artificial spikes after a 1960 incident in New Mexico in which a pipe split along its seam for 8 miles during such a test. No one was hurt.

The September disaster in San Bruno, Eiber said, "demonstrated what could have happened in their spike test. It's not a good practice."

Boyles, the PG&E spokesman, declined to respond to the criticisms, citing the federal investigation into what caused the explosion.

Records problems

The intentional pressure spike was also problematic because questions have emerged since the explosion about whether PG&E knew the real strength of the San Bruno line when it set the maximum gas pressure at 400 pounds per square inch in 1970.

The utility has conceded that its records erroneously showed that the San Bruno section of the pipeline, installed in 1956, had no seams. In fact, federal investigators found, the ruptured portion of the line not only had seams, but was pieced together in several 4-foot-long sections that were constructed to unknown, potentially inferior, specifications.

The National Transportation Safety Board says its investigators are examining the quality of seam welds that held the pipeline together - welds that PG&E did not know existed when it set the pipeline's maximum pressure. The federal agency has not reached a conclusion about what caused the explosion.

The board said last week that "it is critical to know all the characteristics of a pipeline in order to establish a valid MAOP (maximum allowable operating pressure) below which the pipeline can be safely operated. The NTSB is concerned that these inaccurate records may lead to incorrect" maximum pressure levels.

James Hall, a former chairman of the safety board and now an independent pipeline safety advocate, said PG&E's erroneous records about the line could have led the utility to set the maximum pressure level too high.

"If you don't have the records on the pipe, how are you setting the pressure?" Hall said.

Inspection issues

In fact, federal pipeline officials say, the intentional 2008 surge might have had some bearing on the validity of PG&E's subsequent inspection of the line in November 2009, which found no problems in the pipe.

The only pressure figure the government considers relevant, according to the federal pipeline safety agency, is the highest level at which the line was run from about 1997 to 2002, when Congress passed a law requiring regular inspections of pipelines in urban or "high consequence" areas.

That peak becomes a benchmark that could, if exceeded, activate a new inspection.

PG&E officials said Friday that the utility did not run the line above 375 pounds per square inch from 2000 to 2002, but that older records were unavailable.

PG&E has said it kept the San Bruno line's pressure at 375 pounds per square inch because it was connected to three other, weaker lines with that capacity. PG&E pinched off those lines for the 2008 surge, the utility said.

Putting stress on welds

Any surge above the benchmark level - intentional or otherwise - is exactly the type of incident that, under the 2002 law, should have forced PG&E to conduct a high-priority inspection using a technique that could detect weakness in a pipe's welds, federal officials said.

That's because pressure surges can put stress on a line's welds, meaning the line is no longer considered "stable," the federal pipeline safety agency said.

PG&E went forward with the November 2009 inspection using a method suitable mainly for detecting pipe corrosion, not weakness in welds.

That method, called direct assessment, involves researching records on a pipeline, electronically mapping it using ski-pole-like devices, and digging spot-check holes to examine the pipe.

The method considered more reliable for finding weak welds involves shutting down a pipe, pumping high-pressure water into it, and then repairing any damage that materializes. PG&E has avoided using it on most of its gas transmission pipes, citing the inconvenience to customers and cost of shutting down lines.

Nevertheless, PG&E's intentional spike of the line's pressure might have prevented it from using direct assessment in its 2009 inspection, federal officials indicated.

"Direct assessment is not considered a viable assessment method when manufacturing and construction defects are 'unstable,' and therefore would not be permitted under federal regulations," the pipeline safety agency said.

PG&E did not respond to queries about the legality of the 2009 inspection. The utility has consistently defended direct assessment tests as being reliable.

'Reckless enterprise'

Hall, the former National Transportation Safety Board chairman, called the 2008 spike "a reckless enterprise" that was "obviously an exercise for their financial situation, not safety."

"You are dealing with pipe that has been in the ground more than 50 years, it has never had an internal inspection tool in it, has incomplete records, and they now artificially spike the line?" Hall said. "Why would you take such a high-risk action in a high-consequence area?"

Every spike above normal operating pressure presents a risk of disaster in such an old pipeline, Hall said.

"You can roll the dice many times," he said, "before you come up with snake eyes."